Process for preparing cupric ammonium arsenite solutions



Oct. 30, 1951 E. FARBl-:R 2,573,252

PROCESS FOR PREPARING C UPRIC AMMONIUM ARSENITE SOLUTIONS Filed May 24, 1949 s 3 Sheets-Sheet 5 I N VE N TOR. lziafdlz BYM. gouaqtea/ ATTORNEYS Patented Oct. 30, 1951 'PROCESS 4FORPREPARING CUPRIC AMMO- NIUM ARSENITE SOLUTIONS ,-Eduard Farber, Washington, "D. C., rassignorto Timber Engineering Company, Washington, Fl). `C.,.a'corporation of Delaware y"Application May l24, T949, Serial No. 95,082

This Iinvention :relates fto "cupric 'ammonium arsenite wood'impregnatingzsolutions. More :particularly, the invention relates to'aprocessrwlrereby suchsolu-tions may zbe'fprepared fdire'ctly from metallic copper.

Ammoniacal solutions fcontaining icupric -'am-- monium' arsenite have-long :been utilized "as -wood impregnating f-compositions. .These solutions are ofrgreat value :for the .treason that they irapi'd-ly eiect 'penetration tdfthe vwood and, -upon u'evaporation of fvolatile matter 'contained therein, l"deposit :inthe wood fibers toxicfcnpperlarsenite salts which zare essentially water-insoluble iand -Ihence .do not 4'leach "out appreciably i'When the "impregnated woodis exposed to.I moisture. -'Wood'so'impregnatedis'thuse'iectively protected'againstthe mirages of "fungi, termites,` insects, "and .otherldestructive 1 organisms.

"Cupric ammonium arsenite .wood impregnating solutions havebeenivariously prepared by dissolving cupric arsenite inan excess of ammonium hydroxide; by dissolving *both "copper sulfate or a similar cupric salt, and 'arsenious t'oxide in an .excess of ammonium hydroxide; and .'byrst precipitating -fcupric' Ahydroxide "by the -:additiony of fa strong Vbasel to an Y:aqueous solution of Ea cupric salt, removing #the .mother fliquor :and fdissolving the cupric hydroxide and aprede'terminedamount of arsenious oxide in an excess of ammonium'hydroxide. Alloi thesemethods-are disadvantageousmowever, `.in 'that the solutions resultant y'therefrom contain water-soluble .salts lwhich are 'ultimately .deposited in T.the :impregnated Wood 'This .condition .obtains Idespite measures designed to'preclude it such as the aforementioned method which entails tha-initial precipitation and removal of cupric hydroxide from a cupric salt solution. f

As a consequence -of thisfact, Wood impregnated with the solutions resultant from the abovementioned processes demonstrate an undesirably high degree of electrical conductivity.

This fact is of particular significance with respect to wooden poles, cross-arms, and the like, employed to carry electrical wires. Impregnating solutions which appreciably increase the coni Such a method fis 'completely infeasible as a 55 .means for .the.prcductionnf -cupric :hydroxide for usein,thepreparationof.woodlmpregnating-solu tions, however, for the'reason that it entails the use of concentrated 'ammonium 'hydroxide and aeration for a period of at least several hours to effect the reaction of .a -signicant amount of copper.

Now, in accordance with this invention,iit has been discovered that cupric ammonium varsenite wood impregnating solutions may be prepared by contacting with a free oxygen containing gas metallic Vcopper contained `in a diluteaqueous ammoniacal solution-of arsenious oxide-said aqueous ammoniacal solution containing from about one to about three chemical equivalents of dissolved ammonia per liter and from about 0.25 to about v0.6 chemical :equivalents of Vdissolved .arsenious oxide per -Aequivalentof dissolved ammonia.

By -so kadjusting 'the relative proportions` of dis solved arsenious oxide :and dissolved fammonia, .there may be produced by `a'shortfperiodoi'faeration a cupric ammonium arsenite -Wood impregnating solution of the proper concentration.

Inasmuch as relatively inexpensive reactants such as metallic copper Vand .air are employed in the process of this invention, .cupric .ammonium arsenite wood impregnatingsolutions maybe prepared therebyvfar more economically .than bypre viously knownprocesses.

Furthermore, by .virtue of the fact that .cupric ammonium arsenite solutions are preparedin accordance with this invention directly 'from .metallic copper, there is present in the .material deposited by the volatilization of the `ammoniaand water, no appreciable quantity of Water-soluble, ionizable salts.

v.Hence wood impregnated with solutions resultant from the process of this invention is characterized by a remarkably 'low degree .of .electrical conductivity.

.In addition, `the toxic .cupric arsenite salts .de- ,positedfin wood impregnated with ,thesolutions resultantrom the ,process of .this invention are not subject to-,removaltherefromby leaching.

rllhefollowing examples are offeredasspecic embodiments of the process of this invention:

EXAMPLE I A quantity of two normal ammoniumhydroxide was prepared from ithewve normalproduct by dilution with vrequired amounto'f distilled water.

In five equivalentfportons of this'tWo normal ammonium hydroxide contained in-open vessels, Were respectively dissolved,"0,"0`.12,0.5,`0.7 and 1.2 chemical equivalents perliter of ,arsenious oxide.

Into each of :the resultant five f solutions was placed an equal quantity of small particles of copper wire screen. Invall instances, however, there was utilizeda substantial stoichiometric ex- 3 cess of metallic copper with respect to the dissolved arsenious oxide present.

Air was passed through each of the reaction mixtures at the rate of from about 6 to about l5 cubic feet per hour per liter of the original ammoniacal solution for a period of one-half hour.

The reactions were eiected at atmospheric pressure and were initiated at room temperature Table I [All data are in chemical equivalents per liter] Composition of Original Reaction saumon lfl Copper ori iria] Dissolved g C Ratio 39m-l u N Ha A5203 Amos/NH3 Dissolved Attention is invited to-the fact that the above data were obtained by experiments with original `reaction solutions 2 normal with respect to ammonia concentrations. Analogous results are obtained, as the normality of ammonia in the original reaction mixture is varied over the range of from about one to about three. Y

EXAMPLE II Equal portions of 0.6 N, 1.2 N, 1.5 N, 2.0 N, and 43.0 N ammonium hydroxide were prepared by dilution of the N product and placed in open vessels. In each of these veportions of ammonium hydroxide was dissolved such an amount of arsenious oxide that the ratio of dissolved arsenious oxide t0 dissolved ammonia in all instances was 0.25. Y

There was then placed in each of the ve arnmonium-arsenious oxide solutions an equal amount of metallic copper of the same type as `'that employed in Example I and which in all instances constituted a stoichiometric excess with respect to the arsenious oxide present.

The Production of copper ammonium arsenite from each of the five reaction mixtures was then eiected by passingv air therethrough at the same rate and under the same conditions and for the same length of time as in Example I.

The results of the five tests are recorded in Table II.

The data set forth in the foregoing table which are graphically represented in Figure 2, illustrate that the extent and rate of reaction of the metallic copper is a function of the dissolved ammonia concentrationV of the original reaction solution as Well as of the dissolved-arsenious oxide concentration thereof, as shown by Example I.

'I'hese two examples in combination, therefore, demonstrate that the concentration of dissolved ammonia and the concentration of dissolved arsenious oxide are interdependent variables which determine the extent and rate of the reaction by which the desired cupric ammonium l arsenite is produced, and further, that these interdependent variables must be properly adjusted to secure the desired result.

It is further apparent from the data of Table f 1I and from Fig. 2 that the extent of the reaction by which the cupric ammonium'arsenite is formed is not appreciable, evenl in the presence of an optimum concentration of arsenious oxide, until the concentration of dissolved ammonia in the original reaction solution approximates about one equivalent per liter.

Likewise, the table indicates that the rate of reaction of metallic copper increases only grad-V ually as the concentration of dissolved ammonia in the original reaction vmixture is increased beyond about 1.5 equivalents per liter.

It should be noted thatxthe data of this example obtain when the ratio of equivalents dissolved arsenious oxide per liter to equivalents of dissolved ammonia per liter is 0.25.

Analogous results are obtainedfrom the use Y of reaction mixtures in which the relative concentration of arsenious oxide is increased to about 0.6 equivalents per equivalent of dissolved ammonia per liter of original reaction medium.

EXAMPLE In Table 'III Equiva- Time in lents per Hours Liter Copper Reacted The foregoing data illustrate that the reaction by which `the cupric ammonium arsenite solution is formed is essentially complete after aeration for one-half hour. In fact, aeration Vfor an additional half-hour actually decreased the yield, while aeration for two hours effected only an insignificant increase.

EXAMPLE IV A series of experiments were carried out in I5 the manner as described in Example I. In this .attracco .the :relative .amounts of ammonia aand Yarsenious oxide dissolved in thecoriginatreaction y@solutions and theecorresponding.. resultsf obtained aarendicatedf in .Table RIV.

Table v"IV v[All parts are in equivalents per liter] .The zdata of Table .IV'a-re finr'accordance'with the teachings of the Examples I and 1I.

' Thus :the metallic copper '.does r-not;reaet .ap- .preciably in thejpresencefof an;original .reaction mixture which contains 0.6 equivalents of edis- ...solved1ammonia but -no dissolved1arseniousmxide ,11er liter. Nor `does f doubling the concentration of dissolved ammonia eiTect-asignicant .increase .in 'the @extent .of 1such :reaction Likewise, 'the vaddition -ofearsenious oxide torthe original :re- 'factionsolution.containing 0.6f equivalents :of fdisasolved -ammonia per liter -faiis to .eiect the reaction of .an v:appreciable 1 amount 'of `.metallic :691513812 :l-Iowever, -when :the concentration of 1`dissolved ammonia in theoriginal rreac'tionvsolution is fin- '.creased to llfequivalentsiperliterland the .-'conrcentration "of rdissolved :arsenious -zoxide is Iin- .:acreasedto 0.3equivalents'per equivalent :ofzdissolved ammonia, the .'reaction rproceedsrapidly. sFurthermore, athis #desirable `reaction frate is flnaintained and increases .as 4theconcentration of ammonia is increased to about .13 Aequivalents --per liter iand .the `vratio tof "equivalents of fdis- A@solved-.arsenious ioxideftoaequivalentsiof dissolved .ammonia I.is lincreased itc-.0i6.

EXAMPLE `V The ,.procedureof the foregoing ex-fnnple was imodfed -by .the fadditi-on -of .asmall amountof .bor-.ic acidfto-.thatoffthe original solution.

. .Thus -there .were dissolvedin za .portiomof .2 -normal .ammonium hydroxide -.con-tained .in 1an -open vessel 0.5-equivalents per .literofaarsenious @oxide and-0.2 equivalents per literfofeboric acid. Into .this .solution was -;placed .copper Wire -in stoichiometric .excess .with respect to `the arse- .niousoxidefpresent .was -passed through ithe reaction. mixture .at-the rate.of about 7 .cubic .feet ,per .-liter.of fthe .original Areaction solution .for a lperiod T.of about thirty minutes.

.-At .the vtermination Iof the .reaction .,per-iod ea gproduct-containing 0.42 chemical gequivalentsfof copper per liter .was obtained.

.The metallic copper-employed in the j.process of :this einvention may bein any1desired form. Thus, copper .sheets or Wire fmay be utilized. .s1-likewise, copper `in -the form of' filings, turnings, .or `Vother discrete particles fmay .be .employed .Copper plated metal scrap .may be -usedif .desined It preferred, however, .that Atheernetal be in such storm .as fto present sa relatively .'.large surface iarea :to .the `r-.eactontmedium.

Arsenates are gcharacterized :by ea :much ..flesser degree f of 'toxicity fthan fare nrsenites. Hence :it

preclude the -oxidation to .thearsenate formY of .the..arsenious"oxide .employed and of .the cupric ammonium .arsenite produced.

This result is achieved by maintaining at all .times in .the .reaction vmedium a. stoichiometric -excess vof v.metallic copper Vwith respect to the arsenious .material present. .The maintenance of v.such an Aexcess .of vcopper :accordingly constitutes a critical and essential feature of this invention.

.-As indicatednby theexamples, it is also critical .to Athe success .of the process of this .invention .that .the Aoriginal .reaction medium have dissolved .therein at least about one equivalent per liter of ammonia. .Likewise the utilization of a reactionmedium containing more than about three .equivalents per liter of dissolved ammonia is unsatisfactory.

It therefore, .preferred that the original reaction medium contain from about 1 to 3 .equivalents Aof y.dissolved ammonia. -A particularly desirable .concentration .of dissolved ammoniais from .about 1.2 to about 1.5 chemical equivalents per liter. If desired, all, .or a portion .of the requisite ammonia may be introduced into thereaction mixture in the form of a lgas in conjunction With the free oxygen containing gas utilized.

Likewise, asshown in the examples, it is essential lthat the .original .reaction medium contain .in solution at least about 0.25chemical equivalent of arsenious oxide per equivalent of dissolved ammonia. The rate and extent of the vreaction .by which the cupric. ammonium arsenite is formed may be enhanced by .increasing to about 0.6 the ratio of equivalents of dissolved arsenious oxide withrespect to the equivalents of dissolved ammonia per liter of original reaction medium. Itis preferred, however, that the concentration of dissolved 'arsenious oxide fall within the range of from about 0.25 to about 0.3 equivalents per .equivalent of Adissolved ammoniapresent in the original medium.

If desired, the reaction may be initiated in a reaction medium Ycontaining a relatively ksmall proportion of arsenious oxide, and additional arsenious oxide added as the reaction proceeds. .This method is particularly Yadvantageous when relativelylarge amounts of arsenious oxide are employed.

The reaction by which the desired cupric ammonium arsenite isV produced may be initiated by passing 'airyor'other free oxygen containing gas 'through the reaction mixture comprising me- Atalliccopper suspended in an ammoniacal arsenious oxide solution. Essentially pure oxygen, 'or'oxygen diluted With an inert gas such as nitro- #gen may nloeiso employed. Air is preferred.

The aeration of thereaction mixture maybe carried out at any desired rate. Generally speaking, "there .should .be present at all times an kex- .cessfof free oxygen, `that is, the rate of supply :of oxygenshould exceed the rate -at which it is consumed by the dissolved metallic copper. It is apparent I.that the A'most desirable vaeration rate willvary x4with thc-:particular 'oxygen containing 'gas'.femployed.

iptimumfrates"oftaeration are, to some extent, :likewise aifunotion .of Vthe form of ythe metallic 1copper employed- .Thus when the copper .employedipresents zalarge, surface area to the rei'actionzrrfedium, relatively :high :rates of :aeration -may ".be utilized, whereas flow vaeration rates .are :desirable `.when ithe surface 'development of the :.isssential, in T.the Pprocess v of :invention to 15 Luopperxutilized isismall.

In any event, those skilled in the art will readily be able to select the optimum aeration rate for the particular oxygen containing gas and the form of copper employed.

The preferred aeration rate when airis employed as an oxygen containing gas and copper in the form of wire or discrete particles is utilized, is from about 6 to about 10 cubic feet per hour per liter of original reaction solution. l

In any event, it is a signicant feature of this invention that the reaction of an adequate amount of metallic copper is effected by a short period of aeration in the presence of dilute ammonium hydroxide. A preferred period of aeration is from about to about 45 minutes.

Prior art process entailing the oxidation of metallic copper in the presence of aqueous ammonia employs the use of quite concentrated ammonium hyroxide solutions and aeration for a period of at least several hours.

Preferably the reaction is effected at a temperature from about 10 C. to about 40 C. I-IoW- ever, the temperature may be varied somewhat beyond these limits if so desired.

The temperature of the reaction mixture will tend to rise during the course of the reaction as a consequence of the exothermic nature thereof. I'his eiect is of particular significance when the maximum feasible concentrations of ammonia are present in the original reaction medium. Thus when reaction solutions which are about 2 normal with respect to dissolved ammonia are employed, the temperature rise during aeration may be as much as C., whereas in the case of starting solutions which are about 3 normal with respect to dissolved ammonia the temperature rise during the course of the reaction may be as much as C. Operation at temperatures in excess of 40 C. is undesirable as a consequence of the attendant increase in the amount of ammonia lost by evaporation.

Such loss of ammonia may be reduced to some extent, however, by initiating the reaction in the presence of a reaction medium containing relatively small amounts of ammonia and increasing the ammonia to the desired strength during the course of the aeration. K

For example, arsenious oxide may be dissolved in an ammoniacal solution only one normal with respect to ammonia, aeration initiated, and additional ammonia added during the reaction period to form an appropriate solution thereof.

Alternatively, a measured amount of ammonia gas may be admixed with the air introduced into the reaction medium by the aeration system.

If desired, the ammonia lost by volatilization may be recovered by conventional methods, such as scrubbing or by absorbing the ammonia in suitable absorbents.

The process of this invention is particularly advantageous from a commercial standpoint in that it may readily be practiced either as a batch or a continuous operation.

One type of apparatus in which the operation of this invention may be carried out is diagram'- matically represented in Fig. 3. In the gure, I represents a reaction tank equipped with an inlet orice 2 having a closure 3, a gas outlet orice 4, a, perforated bottom insert 5, an aerator 6, and a reaction product outlet 1. The gas outlet orince 4 is connected by means of conduit 8' to chemical make-up tank 9 and by means of conduit I0 to scrubbing tank II. Scrubbingtank II is equipped with water inlets I2 and connected through pump I3 and conduit I4 with chemical make-up tank 9.

In batch operation the reaction tank I is charged with the desired amount of metallic copper through orice 2 which is then closed by means of closure 3. Tank I is then filled through the gas outlet 4 by means of conduit 8 with an aqueous solution of ammonia and arsenious oxide which has been prepared in chemical make-up tank 9.

The reaction is then initiated by passing air or other oxygen containing gas through the reaction mixture by means of aerator B. The ammonia-rich oil gas produced escapes through gas outlet 4 and passes through conduit I0 into scrubbing tank I I where it is scrubbed with water from inlets I2. The resultant ammonia-containing wash waterl is passed by means of circulating pump I3 through conduit I4 into chemical makeup tank 9.`

Although only one scrubbing tank is shown in the gure, several may be employed if so desired.

From the reaction tank IV the copper am monium arsenite solution produced is removed through outlet 'I and the cycle repeated.

The invention may be practiced in a continuous manner in the above-described apparatus by initiating the discharge of the copper ammonium arsenite reaction product about thirty minutes after the reaction has been started and by main--V chemical make-up tank 9 in the form of an' aqueous slurry.

Ammonia gas may be dissolved in Water contained in chemical make-up tank 9 or if desired may be introduced directly into reaction tankl in conjunction with air by means of aerator 6.

Cupric ammonium arsenite wood impregnating solutions prepared in accordance with the process of this invention may contain arsenious and divalent copper ions in concentrations of from about 0.4 to aboutl 0.6 chemical equivalents per liter, and from about two to about four equivalents of dissolved ammonia per chemical equiva` lent of divalent copper present. Cupric `and arsenious ions are customarily produced in approximately stoichiometric proportions.

These cupric ammonium arsenite solutionsv rapidly and effectively penetrate wood and other cellulosic materials and upon volatilization of the ammonia and Water contained therein deposit in the wood fibers toxic residue essentially'freeof water-soluble materials which are remarkably resistant to leaching by moisture. Hence wood impregnated with a solution prepared by the process of this invention is essentially permanently protected against the ravages of termites, insects, and other destructive organisms.

In addition, the wood impregnating solutions Y which attend the process of this invention insofar as the electrical conductivity of the impregnated and dried Wooden products is concerned, a series of comparative tests with the representative wood impregnating solutions of the prior art were carried out.

The prior art impregnating solution tested was prepared with the teachings of U. S. 2,149,284 to Gordon, by dissolving copper sulfate in water, precipitating cupric hydroxide by the addition of sodium hydroxide to the copper hydroxide solution, removing and washing the cupric hydroxide precipitate, dissolving the washed cupric hydroxide in an excess of ammonia and adding to the resultant ammoniacal solution a predetermined amount of arsenious oxide.

Comparative tests with this prior art solution and the solutions prepared by the process of this invention were made by an independent, disinterested laboratory, in the manner described in Example V.

EXAMPLE v1 Pieces of white fir Wood approximately 3 inches long, 2 inches Wide and l inch thiclr were impregnated under identical conditions with each of the solutions to be tested. Approximately equal amounts of both solutions were absorbed by the r wood pieces impregnated.

The' impregnated wood samples were dried after impregnation and conditioned to about 12% moisture. The volume resistivity of the samples was then determined in accordance with ASTM Method D25746. The samples were then maintained at 77 F. and 90% relative humidity for a period of one week and the volume resistivity thereof was again determined. The results of these tests are tabulated in Table IV.

The electrical resistance determinations were made with a general radio type 544-B megohm bridge. Readings were taken after testing for one minute at an applied potential of 500 volts direct current.

It is apparent from the foregoing table that the wood impregnated with the copper ammonium arsenite solutions prepared by the process of this invention demonstrates a much greater resistance to the passage of electrical current and hence a remarkably lower degree of conductivity than does similar wood impregnated with the solutions of prior art.

I claim as my invention:

1. A process for the production of a cupric ammonium arsenite wood impregnating solution substantially free of arsenates which comprises contacting with a free oxygen containing gas at a temperature of about 10 C. to about 40 C., an aqueous ammoniaca] arsenious oxide solution having metallic copper in contact therewith, said aqueous ammoniacal arsenious oxide solution containing from about 1 to about 3 equivalents of dissolved ammonia per liter` and from about 0.25 to about 0.6 equivalent of dissolve-d arsenious oxide per equivalent of dissolved ammonia, said metallic copper being present throughout the reaction in a stoichiometric excess with respect to the arsenious material present, said free oxygen containing gas being supplied at a rate adequate to provide an excess of oxygen throughout the reaction.

2. The process of claim 1 wherein the free oxygen containing gas is air.

3. The process of claim 2 wherein the air is passed through the reaction mixture at the rate of from about 6 to about 10 cubic feet per hour per liter of aqueous ammoniacal solution for a period of from about fifteen to about forty-five minutes.

4. A process of preparing a cupric ammonium arsenite wood impregnating solution substantially free of arsenates which comprises passing a free oxygen containing gas through an ammonical arsenious oxide solution having metallic copper in contact therewith, said solution containing from about 1.2 to about 1.5 chemical equivalents of dissolved ammonia per liter and from about 0.25 to about 0.3 equivalents of dis- Solved arsenious oxide per equivalent of dissolved ammonia, said metallic copper being present in stoichiometric excess throughout the process, said free oxygen containing gas being supplied at a rate adequate to provide an excess of oxygen throughout the reaction, said process being effected at a temperature of about 10 C. to 40 C. for a period of from about 15 to about 45 minutes.

EDUARD FARBER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 884,298 Schaefer Apr. 7, 1908 1,057,076 Monseur Mar. 25, 1913 1,082,658 Somermeier Dec. 30, 1913 2,149,284 Gordon Mar. 7, 1939 2,263,594 Rushton Nov. 25, 1941 FOREIGN PATENTS Number Country Date 782,795 France 1 Dec 17, 1934 

1. A PROCESS FOR THE PRODUCTION OF A CUPRIC AMMONIUM ARSENITE WOOD IMPREGNATING SOLUTION SUBSTANTIALLY FREE OF ARESNATES WHIC COMPRISES CONTACTING WITH A FREE OXYGEN CONTACTING GAS AT A TEMPERATURE OF ABOUT 10* C. TO ABOUT 40* C., AN AQUEOUS AMMONICAL ARSENIOUS OXIDE SOLUTION HAVING METALLIC COPPER IN CONTACT THEREWITH, SAID AQUEOUS AMMONICAL ARESNIOUS OXIDE SOLUTION CONTAINING FROM ABOUT 1 TO ABOUT 3 EQUIVALENTS OF DISSOLVED AMMONIA PER LITER AND FROM ABOUT 0.2K TO ABOUT 0.6 EQUIVALENT OF DISSOLVED ARESNIOUS OXIDE PER EQUIVALENT OF DISSOLVED AMMONIA, SAID METALLIC COPPER BEING PRESENT THROUGHOUT THE 